The present invention is related to systems for measuring the signal-to-noise ratio of a carrier wave and more particularly to a system for providing a carrier wave having a predetermined signal-to-noise ratio.
The signal-to-noise ratio of a carrier signal serves as a figure of merit for the performance of communications systems. As the signal-to-noise ratio increases, it becomes easier to distinguish and to reproduce the modulating, or other signal of interest, without error or confusion. Apparatus for measuring the signal-to-noise ratio of a carrier signal is well known in the art and finds particular application in the communications field and in the electronic testing equipment related thereto. U.S. Pat. No. 4,605,904 entitled "Phase Lock Loop Frequency Demodulator Having an Adjustable Bandwidth", issued to Mohammed Hajj-Chehade on Aug. 12, 1986 discloses a sampling circuit for taking a sample of a demodulated signal from the output of a demodulator at the start of a synchronizing signal at which time the demodulated signal is exclusively composed of noise. The sampling circuit provides a reliable and accurate measurement of the noise at the demodulator output by measuring the noise in the absence of a signal rather than measuring the signal to noise ratio in the presence of a useful signal. U.S. Pat. Nos. 4,630,117 and 3,775,689 provide additional examples of known prior art signal-provide to-noise ratio measuring systems.
One of the problems in the prior art is the lack of a system for providing a precise signal-to-noise ratio standard to calibrate and verify signal-to-noise measurement systems.